Transportation News & Views

How do they do that? Change a flat on a steel train wheel

‘How do they do that?’ is a series for The Source that explores the technology that helps keep Metro running and passengers and other commuters moving. Some of it applies directly to the trains, buses and freeways and some of it runs in the background — invisible to nearly everyone but essential to mobility in our region.

How do they change the steel wheels on the trains? And why? It’s not like they can get a flat. Or can they?

There are 2,884 wheels in the Metro Rail fleet: 2,052 on light rail and 832 on the subway. At some point in their working lives, many of those wheels will need to be “re-profiled” or replaced.

What would cause a steel wheel to wear out? Many of the same things that damage car tires: Sudden stops. Sweeping curves. Lots of miles. While many of us change our car tires every 50,000 miles or so, Metro rail wheels can travel as far as 700,000 miles before they need to be replaced. Good thing because changing the wheels on a single rail car can take more than a week, depending on the design of the car.

Re-profiling a steel wheel is the process of removing a thin layer of the wheel tread and flange with a large “wheel truing” machine (see photo). The truing machine restores the wheel’s roundness, tread taper and flange thickness to create good ride quality and steering.

And yes, steel wheels can get flats … although not the kind you’re thinking of. Flat spots are caused by the wheel locking up during an emergency stop, usually because it has come in contact with grease or oil that has dripped off automobiles crossing the tracks. This slippery spot can cause the metal wheel to slide on the metal rail and this can generate a flat spot on a wheel. The flat is removed by taking a layer of steel off the wheel, using a lathe or a milling machine. Metro’s “wheel truing” machines have 150 cutters on each side that can re-profile two wheels at the same time. When the steel tires are too small to “wheel true” any more, they are replaced.

To change the steel tires on one of Metro’s older light-rail cars, the car is lifted with an in-ground vehicle hoist that is powerful enough to support the 98,000-pound light-rail cars. (The term “light-rail” actually is a misnomer since light-rail cars weigh more than so-called “heavy” rail subway cars, which run around 80,000 pounds.) Once lifted, the entire “truck assembly” — the thing the car sits on that contains the wheels — needs to be disassembled to the point where all that is left is the wheels and the axel assembly. The old steel tires are cut off with a band saw and new tires are pressed onto the old wheel center. Each light-rail car has three trucks and subway cars have two … the reason light-rail weighs more than heavy. To encourage a quiet, cushioned ride, Metro light-rail steel tires are supported by 19 rubber blocks.

Newer light-rail cars — the AnsaldoBreda P2550 cars that run on the Gold Line — have steel tires that can be removed while still on the car. This takes only about one day and is, obviously, a huge labor saving enhancement. The new Kinkisharyo rail cars — the first of which are scheduled for delivery in two years — will have the same feature.

Subway wheels are different from light rail. They do not have tires and are larger and have more metal that can be removed during wheel truing. When the subway wheels are too small to be “trued,” the wheels are removed from the axels and the entire wheel replaced. After repair or replacement, the wheels are re-attached with a gigantic press applying thousands of pounds of pressure because the hole in the wheel is actually smaller than the diameter of the axel. This is called a press fit. No bolts or lug nuts hold on the wheel. Like the older light-rail cars, the process on heavy rail cars takes about three days per truck assembly section.

Metro train wheels are closely monitored using special tools and highly trained employees to inspect and measure the wheels. When necessary, the wheels are “trued” or replaced by specially trained Metro employees to ensure a safe and comfortable ride. Like the wheels on a car, train wheels need to be in top condition for safety, as well as a smooth and comfortable ride. Also like a car, Metro wants the wheels to be in tip-top condition so that on the occasions when a sudden stop is required, they’re ready to do the best possible job to keep us safe.